# Fun with Neutral Pressure Planes (NPP)

Greetings

First of all......

* As we upgrade and build new....

I think it is time to eliminate atmospherically vented appliances from the breathing zones of our homes

* I realize that the effects of wind and mechanical equipment can and often do overwhelm Stack Effect.

My current favorite online resources for visualizing "Stack Effect" are.....

*John Straube's Article

http://www.buildingscience.com/documents/digests/bsd-014-air-flow-c...

*John Klote's Article

http://fire.nist.gov/bfrlpubs/fire91/PDF/f91013.pdf

*Bud Poll's Worksheet

I plan to post some Illustrations and see if you folks agree with my current thinking about the location of Neutral Pressure Planes

Meanwhile...

Does anyone else have suggestions for online links concerning  "Stack Effect"?

Views: 5454

### Replies to This Discussion

Robert, if you live at 10,000-ft, do you think the effect on the NPP from opening an upstairs window will be the same as a home at sea-level?

I don't think the altitude would have much impact on the neutral pressure plane, which is almost entirely a function of the relative size of high and low openings, but it would reduce the stack effect pressure because there would be less air density differential at the same delta-T.

On the other hand, if you have much colder winter temperatures because of the altitude, then your stack effect pressure could be a lot higher.

Stack effect pressure differentials and the location of the NPP are two separate issues.

I was reacting to the formula and while it's easy to assume the NPP would be in the same place, density must matter, at lower density the height gives a differential total pressure, with less pressure the opening sizes will alter values from that via drag coefficients altering and the leakage rates of heat-transfer by less dense air having lower heat-retention, especially drier air.

Just seems it'd alter the NPP and your reply is too vague to answer this specific issue so after more. Adding that in most engineering there's a fudge factor based on empirical data to theory, these are buildings, so, my expectation is that altitude will matter.

Really Robert? ... do you think the atmosphere is the same in physical characteristics at altitude as at sea-level?

This isn't theory, my experience says a building will NOT work at altitude as at sea-level, so are you implying otherwise?

My pisser is radiative heat-loss at night, please explain or give up.

Yes really. I still don't understand what you're trying to get at. Not only did I never suggest such a silly thing as the atmosphere having the same characteristics at high elevations as at sea level, but I've been arguing that elevation and hence atmospheric pressure DOES have a significant impact on stack effect pressure (though not on the NPP).

And I don't have a clue what you mean by "a building will not work at altitude as at sea level". What part of a building's "work" are you referring to.

You're going to have to express yourself a lot more clearly if you want an intelligent answer.

never mind ... later if you don't get what I posted, have a nice day.

We are drifting a little so I'm going back to the top.

Allison,

Thanks for coming back.  Actually you did suggest something you haven't thought of, explaining all of that in plain English.  You suggest I'm continuing to confuse people?

Let me partially quote you if I may:

"Archimedes' Principle .. Sir Isaac Newton and Herr Gottfried Wilhelm Leibniz and the calculus…That's where the calculus comes in. (For every epsilon, there exists a delta...)…infinitesimal point particle."

Tell me, how does that wording help to simplify the explanation of stack effect?

I'm not trying to criticize you, it is just that academics often like to speak and write in a different language.  I find paper after paper related to this topic, it's not new, but they are written at an engineering level.  Last I looked, calculus was not on the energy auditor prerequisite list.

Now, your statement: "The answer comes most simply from Archimedes' Principle, which says the buoyant force on a body in a fluid, which in this case is a less dense fluid, is equal to the weight of the fluid displaced. That's all there is to it really."

That's not actually stack effect, as it encompasses the entire volume of the house.

Now, if you reduce that volume to a defined column (weight in Pascals) of air inside and compare it to a similar column of air outside, whether those columns stops at the top of the house or extend to space, you have the net stack pressure.  Gee, that sounds familiar.

Now, again, continue your explanation to include the Neutral Pressure Plane (NPP) and what actually causes it to shift when we open an upstairs window.  What simple principle allows it to move around?  Take it one step further and tell me how we can use our manometer to locate the NPP?

Bud

The 3 links at the top of this thread are the most "helpful" I have found (to date) ...for visualizing stack effect. (Straube, Klote & Poll)

I would sure welcome more suggestions for other online resources...Allison,anyone else..any other suggestions?

I realize that in the Klote Link....Barometric Pressure is not required as an input in order to estimate the location of the NPP

And in the Straube Link...Barometric Pressure is not required as an input in order to estimate the Total Pressure

The problem I see with the Straube & Klote links and illustrations ... they do not really offer any visual cues or provide a sense of scale to aid in visualizing "stack effect"

I think Bud's Link does offer some visual insight into the "stack effect"... and he recognizes that it is work in-progress.

I am attaching a diagram (in-progress) that represents an extremely tight house with only one window open....where I am attempting to show things in a different format

with the "Pascals" shown sort of like "contours" at 10 Pascal intervals (inside and outside)...

Hi John, I'm always jealous of your diagrams, as mine always look so basic.  Work in progress is definitely correct.

The contour lines are interesting, I will think on how they might add to a diagram.

Bud

this diagram shows 2 equal size windows open

notice how the air from the side of higher absolute pressure "spills" downhill toward the side with lower absolute pressure

Those illustrations ARE awesome, and thought provoking.

Wouldn't the upper spill outside look like an inverted mirror of the lower spill? Spill going from higher pressure to lower pressure, and possibly flowing up as it goes out just as below it flows down as it flows in?

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